For selective separation or removal of a specific component contained in a liquid or gas mixture, a separation method utiiizing a membrane has received increasing attention in recent years. This has already been put to practical use in several fields. For example, it is known that a membrane made of a silicone resin is effective in separating an organic component, i.e., a low molecular weight organic compound such as methanol, ethanol, propanol or acetone from an aqueous solution containing the organic compound, or in separation and concentration of oxygen in air. One of the reasons is that a silicone resin exhibits a high selective permeability for a specific organic compound since its surface tension is small as compared with that of the usual organic resins and, furthermore, its intermolecuar force is small. Another reason is that of various resins, the silicone resin has the highest coefficient of permeation for oxygen and nitrogen, for example. Membranes made of polydimethyl siloxane, polydiphenyl siloxane, polyvinyl trimethyl siloxane and the like have heretofore been known.
Membranes made of such silicone resins, however, generally have a poor mechanical strength. In practical membrane separation, therefore, it is necessary for the thickness of the membrane to be increased to 100 .mu.m or more. As is well known, the permeation flux of a membrane is in inverse proportion to its thickness. Therefore, although the silicone membranes have a high coefficient of permeation for organic compounds and gases, they suffer from a problem that a permeation flux is small, which is a factor determining economic efficienty in the membrane separation.
In order to overcome the above problem, it has already been proposed to form a thin film of a silicone resin on a porous substrate made of an organic polymer such as polysulfone or polyacrylonitrile which is conventionally known as an ultrafiltration membrane to thereby increase the mechanical strength of the silicone resin membrane and also to increase the permeation flux by reducing its thickness as described in, for example, Japanese Patent Application (OPI) Nos. 86684/78 and 82380/79 (the term "OPI" as used herein refers to a "published unexamined Japanese patent application"). In forming a silicone resin thin film on such a porous substrate, it is necessary that an organic solution of the silicone resin be coated and then the solvent be evaporated. Since, however, such a porous substrate generally has a low resistance to organic solvents, organic solvents which can be used in preparation of the silicone resin solution are limited to aliphatic solvents of low dissolving power, such as hexane or heptane. Furthermore, these aliphatic solvents can dissolve only silicone resins having a low molecular weight. Therefore, the mechanical strength of a silicone resin thin film formed on a substrate is not sufficiently high and its thickness is still considerably large. In other words, a silicone resin having a molecular weight of 10,000 or more, which is desirable to use in preparation of a thin film having a high mechanical strength, is soluble only in organic solvents of high dissolving power, such as toluene, ethyl acetate or methyl ethyl ketone, and such an organic solution of the silicone resin cannot be coated on a porous substrate as described above.
For separation of an organic liquid mixture, a semipermeable membrane made of an aromatic polyimide resin and a semipermeable membrane made of a polyimide resin prepared by condensation of 1,2,3,4-butanetetracarboxylic acid or its derivative and an aromatic diamine have been proposed and put to practical use. Solutes which can be separated by such a polyimide resin semipermeable membrane are limited to those having a molecular weight of several thousands or more. Such a polyimide resin membrane cannot be applied to reverse osmosis permitting the separation of a solute having a relatively small molecular weight of from several tens to several thousands, or to membrane separation in the intermediate range between reverse osmosis and ultrafiltration.